Switching power supply units have been widely used as power supply units for efficiently converting a given supply voltage to a lower voltage. In order to smooth the intermittent output voltage arising from switching, a switching power supply unit has at the output end thereof a smoothing coil and an output condenser.
The smoothing coil and the output condenser will delay the output voltage if a load is suddenly changed. Thus, there has been proposed an improved power supply unit adapted to alleviate the delay in the response to a change in load, as disclosed in, for example, Japanese Patent Early Publication H10-98874.
FIG. 1 shows a general circuit arrangement of a power supply unit having an improvement in response to a delay pertinent to conventional circuits. A switching power supply circuit 1 of FIG. 1 performs switching to convert a supply voltage Vin of a power source B such as a battery into a pulsed output having a fixed pulse width, based on the comparison of the output voltage Vo with an internal reference voltage (not shown). The pulsating output is smoothed by a smoothing coil Lo and an output condenser Co, and supply a resultant output voltage Vo to a load 3. Diode D is a flywheel diode.
If this load 3 suddenly increases, the output voltage Vo is lowered. In order to compensate for the delay in recovery of the output voltage Vo back to a predetermined level, a series power supply circuit 2 is connected in parallel with the switching power supply circuit 1. Thus, when the output voltage Vo is suddenly lowered by a sudden increase of the load, a current is promptly supplied to the output condenser Co via the series power supply circuit 2, thereby recovering the output voltage Vo within a short delay time.
In this conventional electric power supply unit, the output voltage can be recovered without an appreciable delay even if a rapid change takes place in the load, provided that the power supply unit has a fixed reference voltage. However, in cases where the reference voltage is lowered to change the output voltage, the conventional electric power supply unit cannot quickly lower the output voltage because the series power supply circuit 2 has only a current-feeding capability.
FIG. 2 shows the reference voltage Vref, output voltage Vo, and load current Io changing in time during a transitional discharge. Suppose now that the reference voltage Vref is dropped from an initial level Vref1 to a final lower voltage Vref2 at time t1, as shown in FIG. 2(a). Then the current Io flowing through the smoothing coil slowly decreases as shown in FIG. 2(c), because the discharge by the load 3 is slow and that a back electromotive force is generated by the smoothing coil Lo. Consequently, the output voltage Vo sluggishly changes from its initial level Vo1 to a final level Vo2 associated with the reference voltage Vref2 over a period T2, as shown in FIG. 2(b). It is noted that the waveform of current Io shown in FIG. 2(c) corresponds to substantial switching of the output transistor of the switching power supply unit.
It is, therefore, an object of the present invention to provide a power supply unit having an output condenser, the unit adapted to change its output voltage quickly to the target voltage defined by a variable output voltage setting.